As one of the control methods of internal combustion engines, there is known torque demand control which determines a manipulated variable of each actuator with toque as a control variable. Japanese Patent Laid-Open No. 2009-299667 describes one example of the control device which performs torque demand control. The control device described in Japanese Patent Laid-Open No. 2009-299667 (hereinafter, a conventional control device) is a control device which performs torque control by control of an air quantity by a throttle, control of an ignition timing by an ignition device, and control of a fuel injection quantity by a fuel supply system.
Incidentally, in addition to the quantity of the air which is taken into a cylinder, an air-fuel ratio is closely related to the torque which is generated by an internal combustion engine. Accordingly, in the conventional control device, the air-fuel ratio which is obtained from the present operation state information is referred to in the process of converting the required torque into a target value of the air quantity. The air-fuel ratio in this case does not mean the air-fuel ratio of the exhaust gas which is measured by an air-fuel ratio sensor, but means the air-fuel ratio of the mixture gas in the cylinder, that is, a required air-fuel ratio.
The required air-fuel ratio is not always constant, and is sometimes positively changed from the viewpoint of the emission performance. In such a case, according to the conventional control device, the target air quantity changes in accordance with change in the required air-fuel ratio, and a throttle opening is also controlled in correspondence with the target air quantity. The movement of the throttle at this time becomes such movement as to cancel out the torque variation accompanying the change of the air-fuel ratio by increase and decrease of the air quantity. That is to say, when the air-fuel ratio changes to a rich side, the throttle moves to the closing side so as to cancel out the increase in torque due to this by decrease in the air quantity. Conversely, when the air-fuel ratio changes to a lean side, the throttle moves to an opening side so as to cancel out the decrease in torque by increase in the air quantity.
However, there is a delay in the response of the air quantity to the movement of the throttle, and the actual air quantity changes late with respect to the change of the target air quantity. The delay becomes more noticeable as the change speed of the target air quantity is higher. Accordingly, in the conventional control device, change of the air quantity is unlikely to catch up with abrupt change of the air-fuel ratio when abrupt change takes place in the required air-fuel ratio. In this case, a deviation occurs between the torque generated by the internal combustion engine and the required torque, and not only torque control with high precision cannot be realized, but also worsening of emission performance can be caused due to unintended variation of the air-fuel ratio as a result.
As is known from the above, the conventional control device can be said to have a room for further improvement in the respect of the precision of realization of the required torque in the situation where the required air-fuel ratio can change.